1. Field of the Invention
The present invention relates to a photodetector device and a method for manufacturing the same, and more particularly, to a photodetector device capable of high speed response at low driving voltages, and a method for manufacturing the same.
2. Description of the Related Art
Optical data communications, i.e., optical interconnections, require optical signal sending and receiving portions, i.e., light emitters to produce an optical signal, and photodetectors to detect an optical signal. As the need for high speed transmission increased, optical interconnections, which are applicable for communications through local area networks (LANs), computer-to-computer, computer-to-peripheral device, board-to-board, and chip-to chip, have replaced conventional copper wire. To meet the requirements for high speed transmission, an optical interconnection requires a rapidly responsive photodetector, which is operable with low voltages.
However, a common photodetector formed of a compound semiconductor, which is used for high response speed, cannot satisfy both the need for high response speed and the need for a low driving voltage in optical interconnections for the following reasons.
For p-i-n photodetectors, which are extensively used, electrons and holes are generated in an intrinsic semiconductor material layer by the absorption of light incident through a light-receiving surface. The response characteristics of a p-i-n photodetector is determined by the duration of time required for the thusly generated electrons and holes to reach n-type and p-type semiconductor material layers and by the capacitance, which is a function of the area of the photo-receiving surface and the thickness of the intrinsic semiconductor material layer. In other words, the higher the transport speeds of the electrons and holes and the lower the capacitance, the higher the response speed.
The critical speed of electrons and holes is 1.0xc3x97105 m/s for a Silicon (Si) layer and 1.4xc3x97105 m/s for a Gallium Arsenide (GaAs) layer. Accordingly, GaAs is the preferred photodetector material, relative to Si, with respect to response speed. However, the difference in critical speed between GaAs and Si is not substantial. Thus, the response characteristics of a p-i-n photodetector are markedly influenced by the capacitance of the device.
One of the determinative factors for the capacitance of a p-i-n photodetector is the intrinsic characteristics of the material used to form the photodetector. For example, a photodetector formed of Si has a lower capacitance than one formed of GaAs due to the relatively low dielectric constant of 11.9 for Si, relative to the dielectric constant of 13.1 for GaAs.
Another factor which determines the capacitance of a p-i-n photodetector is the thickness of the intrinsic semiconductor material layer. The capacitance of the photodetector is inversely proportional to the thickness of the intrinsic semiconductor material layer. In particular, for a general p-i-n photodetector formed of Si, the intrinsic semiconductor material layer must be as thick as 20-25 xcexcm, due to the low absorbency of Si, which has an indirect transition band gap, thereby lowering the capacitance of the photodetector. In contrast, for a p-i-n photodetector formed of GaAs, the intrinsic semiconductor material layer can have a thickness of 3-5 xcexcm, due to the high absorbency of GaAs, which has a direct transition band gap, thereby increasing capacitance.
Thus, a p-i-n photodetector formed of Si, which has a thick intrinsic semiconductor material layer, needs high driving voltages in order to facilitate the migration of holes generated along with electrons by light absorption, which migrate much slower than electrons, thereby increasing the response speed. Accordingly, a p-i-n photodetector formed of Si is inappropriate in application fields which require both a high speed response and a low driving voltage. In contrast, a p-i-n photodetector formed of GaAs has a thin intrinsic semiconductor material layer and thus is operable with a low driving voltage. Accordingly, p-i-n photodetectors formed of GaAs are suitable for application fields which require a low driving voltage. The high capacitance of GaAs photodetectors, however, limits their response speed.
Another factor which determines the capacitance of p-i-n photodetector is the photo-receiving area. Capacitance is proportional to the photo-receiving area, and thus capacitance can be lowered by reducing the photo-receiving area. When a GaAS photodetector is used to receive light in a high-frequency band, the photo-receiving area must be further reduced. However, for this case, a restrictive control of allowable error is required for the alignment with an optical axis, thereby increasing the packing and alignment cost.
FIG. 1 shows an example of a conventional p-i-n photodetector. The photodetector is integrated on a n-type substrate 10 beginning with an n-type semiconductor material layer 21, upon which an intrinsic, i.e., undoped, semiconductor material layer 23 and a p-type semiconductor material layer 25 are stacked in sequence. On the top of the p-type semiconductor material layer 25, an annular p-electrode 27 is formed of metal. Also, an n-electrode 29 is formed on the underside of the substrate 10.
The photodetector has a mesa 20, which is etched around the outside of the p-electrode 27 to a depth which extends just inside the n-type semiconductor material layer 21. The back side of the substrate 10 is lapped to have a desired thickness. In order to avoid the occurrence of dark current, the exposed sidewalls of the mesa 20 are covered with an insulating layer (not shown) or polyimide (not shown). For the conventional p-i-n photodetector, the top of the p-type semiconductor material layer 25 inside the p-electrode 27 serves as a photo-receiving surface 25a. 
Another conventional p-i-n photodetector is shown in FIG. 2. In this case, the p-i-n structure is implemented on an n-type substrate 10 by diffusion. For convenience and clarity, corresponding layers having like structures and functions are denoted by the same reference numerals as in FIG. 1. Reference numeral 26 represents an insulating layer between the p-electrode 27 and the intrinsic semiconductor material layer 23.
In such conventional p-i-n photodetectors, as a reverse bias voltage is applied between the p-electrode 27 and the n-electrode 29, incident light enters the p-type semiconductor material layer 25 through the photo-receiving surface 25a and is absorbed in the intrinsic semiconductor material layer 23 to produce electron and hole pairs. The electrons migrate toward the n-electrode 29, while the holes migrate toward the p-electrode 27, so that a current is output in proportion to the amount of received light.
When Si is used to manufacture p-i-n photodetectors having the above configurations, a high driving voltage is needed, which limits application to, for example, optical interconnection, which requires a low driving voltage. On the other hand, if the above p-i-n photodetectors are manufactured using GaAs, designing a pre-amplifier IC for amplifying the detection signal of a photodetector becomes complicated due to the photodetector""s relatively high capacitance, thereby increasing the manufacturing cost. In addition, when a smaller photo-receiving area is required to receive high-frequency light, the packaging and optical alignment costs increase.
On the other hand, conventional approaches have suggested a resonator type photodetector, as shown in FIG. 3, which is operable with low driving voltages even when the photodetector is manufactured using relatively low absorbency Si. Like a resonator, the conventional resonator type photodetector shown in FIG. 3 includes a first distributed Bragg reflector (DBR) 100 on the p-type semiconductor material layer 25, and a second DBR 101 between the intrinsic material layer 23 and the n-type semiconductor material layer 21. In FIG. 3, the same elements as those of FIG. 1 are designated with the same reference numerals.
In the resonator type photodetector, light incident on the photodetector through the first DBR 100 is repeatedly reflected between the first and second DBRs 100 and 101 and absorbed in the intrinsic material layer 25, to thereby produce electron and hole pairs. The electrons and holes migrate toward the n-type semiconductor material layer 21 and the p-type semiconductor material layer 25, respectively.
Thus, for the conventional resonator type photodetector, even if relatively low absorbency Si is used as a material for the photodetector, the intrinsic semiconductor material layer 23 can be thin, compared to the photodetectors shown in FIGS. 1 and 2. As a result, the low driving voltage requirement can be somewhat satisfied. However, the thin intrinsic semiconductor material layer 23 increases the capacitance, so that the conventional resonator type photodetector is insufficient to satisfy the high response requirement of related application fields, such as optical interconnections.
To solve the above problems, it is an objective of the present invention to provide a photodetector device and a method for manufacturing a photodetector device, which satisfy the needs for low driving voltages and good high-frequency response with a relatively large photo-receiving area, so that the photodetector device and the method can be applied to optical interconnections and other application fields.
According to an aspect of the present invention, there is provided a photodetector device including a doped semiconductor substrate, and a first intrinsic semiconductor material layer formed over the semiconductor substrate. The photodetector device further includes a main reflector formed over the first intrinsic semiconductor material layer, and a second intrinsic semiconductor material layer formed over the main reflector. An upper semiconductor material layer, which is doped with the opposite type dopant as that of the substrate, is formed over the second intrinsic semiconductor material layer. The photodetector device further includes an upper electrode, which is formed on and electrically connected to the upper semiconductor material layer, and a lower electrode, which is electrically connected to the semiconductor substrate. One of the first and second intrinsic semiconductor material layers is formed with a relatively small thickness to absorb incident light, whereas the other is formed with a relatively large thickness.
Preferably, the first intrinsic semiconductor material layer is relatively thick, the second intrinsic semiconductor material layer is relatively thin, and the photodetector device further includes an upper reflector on the upper semiconductor material layer, such that the upper reflector and the main reflector form a resonator structure, thereby increasing light absorption by the second intrinsic semiconductor material layer.
Preferably, the first intrinsic semiconductor material layer is relatively thick, the second intrinsic semiconductor material layer is relatively thin, and the photo-receiving surface of the upper semiconductor material layer and the main reflector form a resonator structure, such that incident light is absorbed in the second intrinsic semiconductor material layer.
According to another aspect of the present invention, there is provided a method for manufacturing a photodetector device, including the steps of preparing a doped semiconductor substrate, and depositing in succession on the semiconductor substrate: a relatively thick first intrinsic semiconductor material layer, a main reflector having a relatively large number of layers, a relatively thin second intrinsic semiconductor material layer, and an upper semiconductor material layer doped with the opposite type to the semiconductor substrate. The method further includes forming an upper electrode, which has a predetermined pattern for electrical contact with the upper semiconductor material layer, on a portion of the upper semiconductor material layer, such that a photo-receiving area of the upper semiconductor material layer is exposed. In addition, the method includes forming a lower electrode on the underside of the semiconductor substrate.
Preferably, the photodetector device manufacturing method further includes forming a lower semiconductor material layer, which is doped with the same dopant type as the semiconductor substrate, between the semiconductor substrate and the first intrinsic semiconductor material layer and/or forming an upper reflector layer having a small number of layers, relative to the number of layers of the main reflector, on the photo-receiving area of the upper semiconductor material layer after the depositing step and before the lower electrode forming step.